Increase In Moisture Transport Research Articles

What Controls the Interdecadal Enhancement in Interannual Variability of Summertime Intraseasonal Precipitation Over South China?

AbstractThis study focuses on the interannual variability of summertime intraseasonal precipitation intensity (SIPI) over South China (SC) in the 30–90‐day range. The results demonstrate a significant increase in the interannual variability of SIPI over SC around the mid‐1990s. By comparing atmospheric and oceanic anomalies between two periods, P1 (1984–1993) and P2 (1994–2003), we identify that both the Silk Road pattern (SRP) and the western Pacific subtropical high (WPSH) play a role in influencing this interdecadal increase in SIPI interannual variability. These system anomalies are modulated by sea surface temperature anomalies (SSTA) in the North Atlantic (NA) and equatorial East Pacific (EP). During years with positive precipitation anomalies in P2, warming EP SSTA induces anomalous Walker cell and local Hadley cell, weakening the WPSH and enhancing convective activity, and increasing moisture transport over SC. The warming NA SSTA near 45°N triggers a more southerly propagation of SRP, thereby providing additional disturbance energy that enhances summer precipitation over SC. Conversely, during years with negative precipitation anomalies in P2, these processes occur in reverse. The enhanced interannual variability of local convective activity from P1 to P2 leads to a corresponding increase in the interannual variability of the WPSH, which, coupled with an amplified interannual variability of SRP, results in heightened interannual variability of SIPI. Additionally, the composite analysis of 30–90‐day ISO events further substantiates the underlying mechanisms driving the interdecadal amplification of SIPI interannual variability. Diagnostic results reveal that the 30–90‐day horizontal wind induces heightened convergence of background moisture over SC.

Orbital modulation of subtropical versus subantarctic moisture sources in the southeast Pacific mid-latitudes

Reconstructing rainfall variability and moisture sources is a critical aspect to understand past and future hydroclimate dynamics. Here, we use changes in the deuterium content of land-plant leaf waxes from two marine sediment cores located off Chile to reconstruct changes in rainfall amount and variation in moisture sources over the last ~50 ka. The records indicate increased moisture in central Chile during precession maxima, but an obliquity modulation is evident in southern Chile. While the southern westerly winds are the dominant factor of precipitation in southern Chile by bringing moisture and perturbations from the extratropics, the subtropics represent an additional moisture source during precession maxima due to a stronger subtropical jet increasing moisture transport from the tropics to the mid-latitudes. These findings imply that a combination of orbital modulation of moisture sources and rainfall amount explains the last glacial moisture maximum and early Holocene moisture minimum in south-central Chile.

Increased Summer Monsoon Rainfall Over Northwest India Caused by Hadley Cell Expansion and Indian Ocean Warming

AbstractFrom 1979 to 2022, the summer monsoon precipitation has increased by a substantial 40% over Northwest India compared to the 1980s. This wetting trend aligns with the future projections of the Coupled Model Intercomparison Project 6 (CMIP6). The observationally constrained reanalysis data indicates that significant sea surface warming in the western equatorial Indian Ocean and the Arabian Sea is likely driving this increase in rainfall by enhancing the cross‐equatorial monsoonal flow and associated evaporation. We demonstrate that the strengthening of the cross‐equatorial monsoon winds is due to the rapid warming of the Indian Ocean and the enhanced Pacific Ocean trade winds, which result from the poleward shift and expansion of the Hadley cell. These strengthened winds boost the latent heat flux (evaporation), leading to increased moisture transport to Northwest India.

Asian summer monsoon responses under RCP4.5 and RCP8.5 scenarios in CESM large ensemble simulations

The response of the Asian Summer Monsoon (ASM) circulation to the Representative Concentration Pathway 4.5 and 8.5 (RCP4.5 and RCP8.5) forcing scenarios is examined using the CESM1 state-of-the-art global circulation model from 2021 to 2050. The projections show that monsoon precipitation will increase over East Asia, the North Pacific Ocean, the Indian Peninsula, and the Bay of Bengal under the RCP4.5 scenario. Conversely, the South Indian Ocean, West Asia, the Middle East, and the Central Pacific Ocean exhibit a decreasing trend in precipitation. Under the RCP8.5 scenario, precipitation is projected to increase over a wider swath of the Indian Ocean and the Middle East Asia. In the RCP4.5 scenario, the low-level wind circulation is likely to strengthen over the entire northern Indian Ocean, extending to the South China Sea, thereby increasing moisture transport from the Indian Ocean to peninsular India and the South China Sea. Conversely, in the RCP8.5 scenario, easterly winds strengthen over the South Indian Ocean, leading to an increase in moisture transport from the equatorial West Pacific Ocean to the Indian Ocean. A weak (strong) cyclonic circulation in response to the east-centered (west-centered) low sea level pressure trend over the North Pacific in RCP4.5 (RCP8.5) scenario is projected to help maintaining a strong (weak) ASM circulation from the India to east Asia. Internal climate variability is also calculated, revealing that the North Pacific Ocean near the Bering Sea is likely to play a dominating role and contribute significantly to the future ASM dynamics. In both scenarios, internal variability is found to substantially contribute to changes in monsoon circulation over the Indian Ocean.

The Decline in Summer Fallow in the Northern Great Plains Cooled Near‐Surface Climate but had Minimal Impacts on Precipitation

AbstractLand management can moderate or intensify the impacts of a warming atmosphere. Since the early 1980s, nearly 116,000 km2 of cropland that was once held in fallow during the summer is now planted in the northern North American Great Plains. To simulate the impacts of this substantial land cover change on regional climate processes, convection‐permitting model experiments using the Weather Research and Forecasting model were performed to simulate modern and historical amounts of summer fallow. The control simulation was extensively validated using multiple observational data products as well as eddy covariance tower observations. Results of these simulations show that the transition from summer fallow to modern land cover led to ∼1.5°C cooler temperatures and decreased vapor pressure deficit by ∼0.15 kPa during the growing season across the study region, which is consistent with observed cooling trends. The cooler and wetter land surface with vegetation led to a shallower planetary boundary layer and lower lifted condensation level, creating conditions more conducive to convective cloud formation and precipitation. Our model simulations however show little widespread evidence of land surface changes effects on precipitation. The observed precipitation increase in this region is more likely related to increased moisture transport by way of the Great Plains Low Level Jet as revealed by the ERA5 reanalysis. Our results demonstrate that land cover change is consistent with observed regional cooling in the northern North American Great Plains but changes in precipitation cannot be explained by land management alone.

Hydroclimatic anomalies detected by a sub-decadal diatom oxygen isotope record of the last 220 years from Lake Khamra, Siberia

Abstract. Northern latitudes have been significantly impacted by recent climate warming, which has increased the probability of experiencing extreme weather events. To comprehensively understand hydroclimate change and reconstruct hydroclimatic anomalies such as drought periods, appropriate proxy records reaching further back in time beyond meteorological measurements are needed. Here we present a 220-year (2015–1790 CE), continuous, stable oxygen isotope record of diatoms (δ18Odiatom) from Lake Khamra (59.99° N, 112.98° E) in eastern Siberia, an area highly sensitive to climate change and for which there is a demand for palaeohydrological data. This high-resolution proxy record was obtained from a 210Pb–137Cs-dated sediment short core and analysed to reconstruct hydroclimate variability at a sub-decadal scale. The interpretation of the δ18Odiatom record is supported by meteorological data, modern isotope hydrology and geochemical analyses of the same sediment, which is indicative of the conditions in the lake and catchment. A comparison with meteorological data going back to 1930 CE revealed that the δ18Odiatom record of Lake Khamra is primarily influenced by regional precipitation changes rather than the air temperature. We identified winter precipitation, which enters the lake as isotopically depleted snowmelt water, as the key process impacting the diatom isotope variability. We related the overall depletion of δ18Odiatom in recent decades to an observed increase in winter precipitation in the area, likely associated with the global air temperature rise, Arctic sea ice retreat and increased moisture transport inland. Available palaeoclimate proxy records, including a fire reconstruction for the same lake, support the idea that the new record is a valuable hydroclimate proxy that is indicative of precipitation deficits and excludes solar insolation and air temperature as primary driving forces, even before the first meteorological recordings. We propose two possible hydroclimatic anomalies that were detected in the Lake Khamra δ18Odiatom record: one at the beginning of the 19th century and a second prominent event in the 1950s. Both are interpreted as prolonged dry periods associated with enriched δ18Odiatom values likely caused by reduced winter precipitation, which coincide with phases of reconstructed severe wildfires in the region. Despite the apparent pristine lake area, we observed a three- to fourfold increase in mercury concentrations and accumulation rates within the sediment record since the early 20th century, which is partly attributed to human air pollution.

Atmosphere and ocean energy transport in extreme warming scenarios

Extreme scenarios of global warming out to 2300 from the SSP5-8.5 extension scenario are analyzed in three state-of-the-art climate models, including two models with climate sensitivity greater than 4.5°C. The result is some of the largest warming amounts ever seen in simulations run over the historical record and into the future. The simulations exhibit between 9.3 and 17.5°C global mean temperature change between pre-Industrial and the end of the 23rd century. The extremely large changes in global temperature allow exploration of fundamental questions in climate dynamics, such as the determination of moisture and energy transports, and their relation to global atmosphere-ocean circulation. Three models performed simulations of SSP5-8.5 to 2300: MRI-ESM2-0, IPSL-CM6A-LR, and CanESM5. We analyze these simulations to improve understanding of climate dynamics, rather than as plausible futures. In the model with the most warming, CanESM5, the moisture content of the planet more than doubles, and the hydrologic cycle increases in intensity. In CanESM5 and IPSL-CM6A-LR nearly all sea ice is eliminated in both summer and winter in both hemispheres. In all three models, the Hadley circulation weakens, the tropopause height rises, and storm tracks shift poleward, to varying degrees. We analyze the moist static energy transports in the simulations using a diffusive framework. The dry static energy flux decreases to compensate for the increased moisture transport; however the compensation is imperfect. The total atmospheric transport increases but not as quickly as expected with a constant diffusivity. The decrease in eddy intensity plays an important role in determining the energy transports, as do the pattern of cloud feedbacks and the strength of ocean circulations.

The Gulf Stream Front Amplifies Large-Scale SST Feedback to the Atmosphere in North Atlantic Winter

The Gulf Stream (GS) ocean front releases intense moisture and heat to the atmosphere and regulates storm tracks and zonal jets in winter. The large-scale sea surface temperature (SST) anomaly in the central North Atlantic provides important feedback to the atmosphere in winter, but the role played in this feedback by the GS front inside the SST anomaly has not been extensively studied. In this study, two sets of ensemble experiments were conducted using a global community atmosphere model forced by SST in boreal winters from 2000 to 2013. The regional averaged SST and its variation in the experiments were identical, with the only difference being the strength of the SST front in the GS region. The large-scale SST anomaly in the central North Atlantic in our model provides feedback to the atmosphere and excites a wave train that extends across Eurasia. With the inclusion of the strong GS front, the first center of the wave train in the North Atlantic is strengthened by approximately 40%, and the wave activity flux toward downstream is highly intensified. When the large-scale SST anomaly is combined with a strong GS front, greatly increased water vapor is released from the GS region, resulting in a 50% increase in moisture transport toward Western Europe. In this scenario, precipitation and diabatic heating both increase greatly on the western Scandinavian Peninsula. With the release of deep diabatic heating, a strong upward wave activity flux is triggered, and the wave train excited by the large-scale SST variation is significantly intensified. These findings suggest that the strong SST front in the large-scale SST anomaly in the central North Atlantic significantly amplifies its feedback to the atmosphere in winter.

Anthropogenic warming induced intensification of summer monsoon frontal precipitation over East Asia.

Summer monsoon frontal rainfall in East Asia (EA) is crucial for water resources and flood hazards in densely populated areas. Recent studies have documented the increasing intensity of summer frontal rainfall over recent decades. However, the extent of ongoing climate change on the intensification of the EA frontal precipitation system remains uncertain. Using an objective method for detecting frontal systems, we found a 17 ± 3% increase in observed frontal rainfall intensity during 1958 to 2015. Climate model simulations with and without greenhouse gases suggest that anthropogenic warming plays a key role in the intensification of EA summer frontal precipitation by 5.8% from 1991 to 2015. The analysis highlights that enhanced water vapor convergence and reinforced western North Pacific subtropical High collectively increased moisture transport to the region, resulting in intensified EA frontal precipitation. The results lend support to the anthropogenic warming-induced enhancement of the EA frontal precipitation and its persistence in the future.

Heat health during future summers in eastern Asia: The combined roles of circulation, temperature and humidity

AbstractExcessive relative humidity (RH) in combination with high temperature can lead to heat stress, often measured by the Wet Bulb Globe Temperature (WBGT). The Clausius‐Clapeyron (CC) relationship implies that warming reduces RH if no extra moisture is added. Over coastal regions like eastern Asia, however, the predominant summer monsoon favours increased moisture transport from surrounding oceans as a result of enhanced evaporation driven by surface temperature increase. This would lessen the RH reduction by potentially two‐thirds. Based on two ensembles of climate model simulations, this paper examines the competing effects of near‐surface temperature, humidity and circulation patterns in this region and the consequential health risks. Under a high emissions scenario (RCP8.5/SSP5‐8.5), surface temperature could increase by 4 – 7°C with WBGT increases of several degrees by the end of the 21st century. Devastating extreme heat health events could therefore become a frequent occurrence as a result. Overall, our results show how humidity can be just as important as temperature when considering the risks to society of excessive heat.

Modeling the effects of global cooling and the Tethyan Seaway closure on North African and South Asian climates during the Middle Miocene Climate Transition

The Middle Miocene was a period of prominent climatic change, marked by the Mid-Miocene Climatic Optimum (MMCO) and the subsequent global cooling due to a decline of the atmospheric CO2 concentrations (pCO2). In addition to this, the closure of the Tethyan Seaway driven by the Arab-Eurasia collision also had an important effect on the paleoclimatic changes during this period. In this study, we use the Community Earth System Model 1.2.2 (CESM 1.2.2) to simulate the effects of global cooling (i.e. pCO2 decline) and the closure of the Tethyan Seaway on the North African and South Asian climates. Our results show that the global cooling led to a precipitation decrease over both North Africa and South Asia, whereas the closure of the Tethyan Seaway resulted in a precipitation decrease over North Africa but an increase over South Asia. The opposite effects over North Africa and South Asia are due to an increased moisture transport from North Africa to South Asia induced by stronger summer atmospheric circulation when the Tethyan Seaway is closed. We further show that the reconstructed records of drying conditions over North Africa during the warming period from the late Early Miocene to the early Middle Miocene from previous studies can be partly explained by the narrowing of the Tethyan Seaway and its climatic continuing deterioration due to the subsequent final closure and global cooling. Both are precursory conditions for the formation of the Sahara desert. The stronger South Asian monsoon during the Middle Miocene transient cooling period found in previous studies can be partially attributed to the final closure of the Tethyan Seaway.

Identifying Eastern US Atmospheric River Types and Evaluating Historical Trends

AbstractAn atmospheric river (AR) is the primary moisture transport forcing in the Western United States, making ARs the predominant producer of extreme precipitation events in this region. A growing body of evidence suggests similar impacts for the Central and Eastern US This study determines the most prominent types of ARs in the Central and Eastern US study domain through the implementation of a machine learning methodology. Self‐organizing maps (SOMs) are leveraged to determine what “flavors” of ARs exist in the study domain. Four atmospheric river detection criteria are utilized to investigate the variability AR types. Mann‐Kendall trend analyses on AR strength and size are produced to evaluate changes over the study period. The results confirm extratropical cyclones as the most common driver of ARs, however, limited kinematic forcing can also instigate the development of AR events. Results show coastal cyclones and lee‐side cyclones are responsible for producing the strongest ARs. The trend analysis results suggest that ARs associated with Nor'easters and ARs originating in the Gulf of Mexico are exhibiting increasing trends in intensity and/or size. Increasing moisture transport by mature cyclones across the Central and Eastern US have important implications for flooding in highly populated corridors. Areas of concern include the Northeast and Southeast US, while localized enhancement of rainfall is seen along the eastern and southern slopes of the Appalachian Mountains. In addition to the physical findings, this research highlights the importance and sensitivity of statistically significant results to the specific atmospheric river detection criteria that was leveraged.

The East Asian Summer Monsoon Response to Global Warming in a High Resolution Coupled Model: Mean and Extremes

Current climate models still have considerable biases in the simulation of the East Asian summer monsoon (EASM), which in turn reduces their reliability of monsoon projections under global warming. We hypothesize that a higher-resolution coupled climate model with atmospheric and oceanic components at horizontal resolutions of 0.25° and 0.1°, respectively, will better capture regional details and extremes of the EASM. Present-day (PD), 2 × CO2 and 4 × CO2 simulations are conducted with the Community Earth System Model (CESM1.2.2) to evaluate PD simulation performance and quantify future changes. Indeed, our PD simulation well reproduces the climatological seasonal mean and intra-seasonal northward advancement of the monsoon rainband, as well as climate extremes. Compared with the PD simulation, the perturbed CO2 experiments show an intensified EASM response to CO2-induced warming. We find that the precipitation increases of the Meiyu-Baiu-Changma band are caused by comparable contributions from the dynamical and thermodynamical components in 2 × CO2, while they are more driven by the thermodynamical component in 4 × CO2 due to stronger upper atmospheric stability. The regional changes in the probability distribution of the temperature show that extreme temperatures warm faster than the most often temperatures, increasing the skewness. Fitting extreme precipitation values with a generalized Pareto distribution model reveals that they increase significantly in 4 × CO2. Changes of temperature extremes scale with the CO2 concentrations over the monsoon domain but not for precipitation extreme changes. The 99th percentile of precipitation over the monsoon region increases at a super Clausius-Clapeyron rate, ~ 8% K–1, which is mainly caused by increased moisture transport through anomalous southerly winds.

Association between recent U.S. northeast precipitation trends and Greenland blocking

AbstractSince the early 2000s, the northeastern region of the United States (USNE) has received increased total annual precipitation along with more frequent extreme precipitation events. Although previous work has discussed the contribution to increased extreme precipitation from tropical cyclones, the large‐scale driver(s) of summer precipitation increases in the extratropics has received little attention. Here, we show that the summer‐season rainfall surpluses across the USNE are related to the increased frequency of atmospheric blocking over Greenland and the negative phase of the North Atlantic Oscillation. The occurrence of these patterns in summer has been previously connected with southward shifted storm tracks and wet conditions across the eastern North Atlantic. Over the USNE, the circulation shifts are also related to enhanced rainfall due to southerly wind anomalies and increased moisture transport into and vertical motion over the region. It is important to note that the current generation of climate models used for future projections is unable to reproduce the observed tendency towards increased atmospheric blocking over Greenland. Thus, clarifying the association between Greenland blocking and recent precipitation changes across the USNE may help inform future climate projections of summer season rainfall for the region.

Influence of the Indian Ocean Dipole on the Large-Scale Circulation in South America

AbstractThe influence of each phase of the Indian Ocean dipole (IOD) on the large-scale circulation in South America is investigated using rainfall observations, fully coupled, large-ensemble, historical simulations (LENS), and forced experiments using the coupled model’s atmospheric component. IOD events often occur when El Niño–Southern Oscillation (ENSO), the largest source of interannual variability of precipitation in South America, is active. To distinguish from effects of ENSO, only cases during neutral ENSO conditions are analyzed in LENS and observations. During the positive IOD polarity, a perturbation in the local Walker circulation leads to increased convection over equatorial South America, resulting in wet anomalies in the Amazon basin. This signal is the opposite of what is expected during El Niño events. Tropical convection anomalies in the Indian Ocean also force an extratropical Rossby wave train that reaches subtropical South America. During positive IOD, the moisture flux from the Amazon to central and southeastern Brazil weakens, resulting in a drying of the area associated with the South Atlantic convergence zone. Meanwhile, the South Atlantic subtropical high strengthens, contributing to a drying in southeastern Brazil. During negative IOD, the induced wave train from the Indian Ocean leads to increased moisture transport to the La Plata basin, leading to wet anomalies in the region.

Efflorescence of microwave-heated alkali-activated cement synthesized with ultrafine coal combustion ashes

Using coal combustion ashes to synthetize alkali-activated cements (AACs) has significant advantages in the sustainable development of coal industry and exploiting cementitious materials for special occasions. Microwave heating and ultrafine precursors have been reported to improve the reaction extent that would greatly influence the strength development at the early ages of AACs, however, few researches focused on their influence on efflorescence, which is caused by the excessive unreacted alkaline in AACs diffusing to its surface, and reacting with CO2 in the air to form white carbonate deposits on the surface, and efflorescence may lead powder and peel on the AAC surface and gradually lose strength from the outside to the inside. In order to investigate the influence of microwave heating and ultrafine precursors on efflorescence, AAC samples adopting normal and ultrafine coal combustion ashes, and heated with steam and microwave respectively were synthetized. Additionally, the properties including geo-polymerization products, pore structure and moisture transport process which directly affect efflorescence were also compared. The results revealed that AACs heated with microwave exhibited faster and more serious efflorescence, especially for precursors with more amorphous phase and higher geo-polymerization reactivity, which can be attributed to the weak crystallization of reaction products improving the instability of sodium ions, and a less compact microstructure dominated absolutely by capillaries that promotes moisture movement, and also the transmission of sodium ions. Faster efflorescence tendency also revealed on AACs synthetized with ultrafine coal combustion ashes, however, their damage was significantly lower than that of microwave heating, and the increased moisture transport rate caused by the unitary capillaries distribution concentrated at 30 nm–100 nm may be the key reason. These results enrich the research on the application of coal combustion ashes and the influencing factors of AAC efflorescence.

Effect of ultrasound pre-treatment on the kinetics and thermodynamic properties of guava slices drying process

The present work aims to evaluate the influence of sonication pre-treatment on the drying kinetics of guava circular slices in electric oven. Circular guava slices with 6.0 mm thickness were pre-treated with ultrasound for 0 min, 10 min, and 20 min, respectively. Drying kinetics experiments were performed at the temperatures of 50, 70, and 90 °C using an electric oven at 1200 W. Drying kinetics experimental data were fitted using empirical models and a diffusive model with third type boundary conditions. Process activation energy and thermodynamic properties were also determined. Drying of pre-treated samples revealed higher drying rates and less process time. Both Page and the diffusion models were adequate to describe the drying phenomena presenting coefficients of determination higher than 0.99 and low chi-squared values. Results also revealed that sonication times increased moisture transport rates with uniform distribution along with the interior of the slices during drying. The activation energy and enthalpy presented negative values and a positive correlation with the sonication time, while entropy and Gibb's free energy presented positive values and a negative correlation to the sonication time.

Strong Summer Atmospheric Rivers Trigger Greenland Ice Sheet Melt through Spatially Varying Surface Energy Balance and Cloud Regimes

ABSTRACTMass loss from the Greenland Ice Sheet (GrIS) has accelerated over the past two decades, coincident with rapid Arctic warming and increasing moisture transport over Greenland by atmospheric rivers (ARs). Summer ARs affecting western Greenland trigger GrIS melt events, but the physical mechanisms through which ARs induce melt are not well understood. This study elucidates the coupled surface–atmosphere processes by which ARs force GrIS melt through analysis of the surface energy balance (SEB), cloud properties, and local- to synoptic-scale atmospheric conditions during strong summer AR events affecting western Greenland. ARs are identified in MERRA-2 reanalysis (1980–2017) and classified by integrated water vapor transport (IVT) intensity. SEB, cloud, and atmospheric data from regional climate model, observational, reanalysis, and satellite-based datasets are used to analyze melt-inducing physical processes during strong, >90th percentile “AR90+” events. Near AR “landfall,” AR90+days feature increased cloud cover that reduces net shortwave radiation and increases net longwave radiation. As these oppositely signed radiative anomalies partly cancel during AR90+events, increased melt energy in the ablation zone is primarily provided by turbulent heat fluxes, particularly sensible heat flux. These turbulent heat fluxes are driven by enhanced barrier winds generated by a stronger synoptic pressure gradient combined with an enhanced local temperature contrast between cool over-ice air and the anomalously warm surrounding atmosphere. During AR90+events in northwest Greenland, anomalous melt is forced remotely through a clear-sky foehn regime produced by downslope flow in eastern Greenland.

An updated biomization scheme and vegetation reconstruction based on a synthesis of modern and mid-Holocene pollen data in China

This paper presents an updated Taxa-PFTs-Biome scheme, based on 18 biomes, 30 Plant Functional Types (PFTs) and 590 pollen taxa from 1630 surface pollen sites in China. The biomization results reveal a total of 16 biome types comprising eight new biomes. By comparing the simulated biome types with modern potential vegetation, 80% of the sampling sites were correctly assigned, which is considerably more accurate than studies which have used previous Chinese biomization schemes. The biome reconstruction for the mid-Holocene (6 ± 0.5 14 C kyr B.P.) shows that: (1) In eastern China, tropical, subtropical, warm-temperate and cool-temperate forests did not significantly expand northward, whilst cold-temperate forests were absent from northeastern China; and (2) in central China, the western boundary of steppe vegetation was shifted westwards due to a ~150–300 km extension of the forest and forest-steppe biomes, which is close to the present-day 200–300 mm isohyet. The northwestward shift of the forest and forest-steppe in central China during the mid-Holocene indicates that moisture levels were enhanced compared to the present day. The climatic mechanism responsible was likely a strengthened East Asian summer monsoon, which resulted in increased moisture transport from the ocean to the semiarid regions in northern China, promoting the development of forest and forest-steppe vegetation. • An updated taxa-PFTs-Biome scheme, based on 1630 surface pollen sites in China, was established. • The average accuracy of the updated scheme is more than 80%, which higher than previous Chinese schemes. • The boundary of the forest-steppe zone during mid-Holocene generally advanced northwestwards by about 200-300 km.

Modulation of the impacts of Madden–Julian Oscillation on winter rainfall in China by El Niño‐Southern Oscillation

AbstractThis study revealed the modulation of the impacts of Madden–Julian Oscillation (MJO) on winter rainfall in China by El Niño‐Southern Oscillation (ENSO) using reanalysis data and statistical analysis. When MJO is active over the Indian Ocean, its impacts are strengthened by El Niño, leading to enhanced positive rainfall anomalies in eastern China. However, the effects of MJO are counteracted by La Niña; thus, significant positive rainfall anomalies exist only in Zhejiang Province and adjacent areas. MJO activity over the western Pacific induces opposite rainfall anomalies in South China between El Niño and La Niña winters; rainfall in South China increases (decreases) in El Niño (La Niña) winters. Such differences in rainfall anomalies are closely related to anomalous circulation and moisture transport. Anomalous southwesterlies in eastern China are enhanced in El Niño winters when MJO is active over the Indian Ocean, which leads to increased moisture transport towards China and stronger ascending motion in eastern China. But the anomalous southwesterlies are weakened in La Niña winters, thereby resulting in less moisture transport and convergence. When MJO is active over the western Pacific, anomalous northeasterlies in mainland China are enhanced (weakened) in La Niña (El Niño) winters, which strengthens (weakens) the moisture divergence and descending motion in China. Moisture convergence anomalies that explain the rainfall anomalies in ENSO winters are mainly attributed to the effects of low‐frequency background state (LFBS) moisture and meridional winds. It was found that ENSO modulates the impacts of MJO by two main ways. First, ENSO can regulate the intensity and distribution of MJO activity. Therefore, MJO in the same phase will lead to different circulation and moisture transport anomalies in China. Second, ENSO‐induced LFBS circulation and moisture anomalies can also influence the moisture convergence in China, which will strengthen or weaken the effects of MJO.